One of the primary neuropathological lesions in Alzheimer's Disease (AD) is the neurofibrillary tangle (NFT). NFT are composed of aggregates of paired helical filaments (PHF) which contain abnormally phosphorylated tau as one of their major constituents. Tau proteins are soluble proteins that bind to and stabilize microtubules. Soluble tau ((tau(s) is likely to be the precursor for formation of PHF-associated tau (tauPHF). However, the immediate, soluble precursors of tauPHF, the effect of these abnormal tau proteins on microtubule function, and the relation between PHF and NFT are not yet known in detail at the level of protein chemistry. Therefore, populations of soluble tau will be characterized from both AD and non-AD control brains, using quantitative immunodot and immunoblot techniques. These experiments will determine what percent of the abnormally phosphorylated tau is in soluble fractions and will determine if tau(s) from AD cases display the same ratio of tau isoforms as PHF. Preliminary studies indicate that the monoclonal antibody, PHF-1 can recognize a small population of tau(s) that may represent phosphorylated intermediates for the generation of tauPHF. Therefore, PHF-1 reactive tau(s) will be isolated from normal and AD cases and their biochemical properties (molecular weight, isoelectric point, immunochemical properties, phosphate content and tau isoforms) will be compared to other tau(s) proteins and to tauPHF. Further experiments will determine if these abnormal tau proteins maintain the capacity to bind to tubulin or modify microtubule assembly/disassembly. The properties of NFT will then be compared to those of less aggregated PHF. Properties of NFT-associated tau which can be studied in NFT of currently available purity include determination of the molecular weight, isoelectric point, immunochemical properties and sensitivity to phosphatases. PHF and NFT of higher purity will be prepared by modifications of techniques previously developed by the PI. In these more purified preparations, the phosphate content of tauNFT and tauPHF will be compared. The preparations of higher purity will also be used to determine if NFT contain non-tau proteins that are not present in less aggregated populations of PHF. These studies will provide knowledge of the protein composition of PHF and NFT and of their soluble precursors, which is essential to a full understanding of the mechanisms involved in the formation of materials which are among the neuropathologic hallmarks of AD.